1. Field of the Invention
This invention relates to a filtering apparatus for the purpose of filtering slurry, and more particularly to a filtering apparatus that includes a stirring apparatus and an output aperture on the top of a filter housing.
2. Description of Related Art
As the integration of semiconductor devices increases, the size of the metal oxide semiconductor (MOS) transistor is reduced accordingly. This, in turn, increases the requirements of interconnects. Therefore, a design including at least two metal layers is gradually becoming necessary for most integrated circuits. In this kind of design, in order to more easily fabricate devices and transfer circuit patterns more precisely, it is important to planarize the wafer, which is usually not even due to the many structured layers formed on it. Furthermore, planarizing the wafer is the main factor affecting the precision of the alignment. A poor planarization causes imprecise alignment of a mask to the wafer and also induces a higher rate of error in fabrication.
So far, chemical-mechanical polishing (CMP) is the only process capable of globally planarizing a very large scale integration (VLSI) structure or even for an ultra large semiconductor integration (ULSI) structure. CMP uses a machine similar to a knife grinder along with a chemical reagent to mechanically grind the uneven profile of the wafer so that it is planarized.
The reagent is usually referred to as a slurry in CMP. Slurry usually includes a solution mixed with silica in colloidal phase or materials in dispersed phase such as aluminum, KOH or NH.sub.4 OH. The grinding particles are extremely hard and have a diameter of about 0.1-0.2 .mu.m. Basically, these particles are used to polish the wafer surface. Moreover, before slurry is transported to the polishing pad to polish the wafer, slurry has to be filtered by a filtering apparatus to protect the wafer from being scratched by impurities in the slurry or even from being seriously damaged, which would result in fabrication failure.
FIG. 1 is a cross sectional view schematically illustrating a conventional filtering apparatus. Referring to FIG. 1, the filtering apparatus is mounted on a CYBEQ IP8000 CMP machine.
A filter 12 is assembled inside a filter housing 10 for the purpose of filtering slurry, which is supplied to the filter housing 10 by a slurry supplying machine 14 through a duct 16 connected to an input aperture 17. The filter 12 includes an aperture 19 on a bottom 18, which allows filtered slurry to flow to a polishing pad 22 through a duct 20.
When the polishing pad is not in use, the slurry is not supplied and is at an idle status. Because slurry is not flowing, slurry is deposited on the bottom 18 of the filter housing 10 and becomes a crystalline deposit 24 and blocks the filter 12. Besides of this blockage problem, slurry is continuously supplied during performance of CMP. Furthermore, because slurry flows out of the filter housing 10 from the bottom 18 through the aperture 19, slurry is poorly filtered, which increases the possibility of scratching the wafer.
Following is a summary of conventional filtering apparatus drawbacks:
1. When slurry is not supplied and is at an idle status, slurry is deposited on the bottom 18 of the filter housing 10 in a crystalline deposit 24 and blocks the filter 12. PA1 2. During CMP fabricating performance, slurry flows out of the filter housing 10 from the bottom 18 through the aperture 19, in which process the slurry is poorly filtered. This increases the possibility of scratching the wafer.